Accurate reflectometry for extreme-ultraviolet lithography at the National Institute of Standards and Technology

Abstract

The most demanding application of extreme ultraviolet multilayer optics is in lithography. The optics in current alpha-class tools are large, and the multilayer coatings must have both optimized reflectance and extremely high uniformity. At the National Institute of Standards and Technology we have recently commissioned a very large sample chamber at the Synchrotron Ultraviolet Radiation Facility storage ring. This dedicated reflectometer is capable of carrying out complete surface maps of alpha- and beta-class tools for the extreme-ultraviolet lithography community. We have conducted a rigorous characterization of the system by varying the output spectrum of our storage ring in conjunction with multiple filter and detector combinations. This exercise has resulted in an accurate model of the effects of out-of-band radiation and scattering on the measured reflectivity. The model can be directly applied to raw data measured under any typical operating conditions to reduce the uncertainty of our measurements to 0.3%. We will report the findings of the characterization along with the resulting model and sample measurements, which show that the requisite accuracy and precision in reflectivity required for beta-class tools has been achieved. We will also present measurements of large-sized curved multilayer samples. Finally, we will report on an upgrade to the beamline’s monochromator, which is currently in progress. This will improve the stability that will yield the wavelength precision needed for all planned extreme-ultraviolet lithography tools.